In higher-quality automotive vehicles, interior lining components, such as instrument boards or steering wheel caps behind which an airbag module is stored, are frequently made of several layers. Generally, the lowermost layer that faces the airbag module is a rigid support layer which gives the lining component its shape, in most cases made of a plastic material, e.g., polypropylene (PP) with or without fiberglass reinforcement, or a wooden molding material manufactured by means of a primary shaping method or by means of a reshaping method. This support layer, either indirectly via an intermediate layer or directly, is joined to a decorative layer.
The intermediate layer and the decorative layer serve to impart an overall visual and tactile texture indicative of the standard of the automotive vehicle involved.
The intermediate layer is a soft, compressible layer of low density, e.g., made of an expanded plastic material or a fiber knitted fabric.
As a rule, the decorative layer is a firm, wear-resistant layer and can be made of a plastic material, e.g., thermoplastic polyolefin (TPO), polyurethane (PU) or polyvinyl chloride (PVC), leather or a textile material.
The selection of the type of material to be used for each layer is mainly based on esthetic considerations.
To ensure that on deployment of the airbag module, an opening is reliably formed, through which opening the airbag can inflate without obstruction into the passenger compartment, a predefined tear seam line is produced,
according to the prior art, in the lining component of the automotive vehicle by means of local removal of material, the outline of which predefined tear seam line describes one or more opening flaps that form the airbag cover.
In case of emergency, it must be possible to rupture this predefined tear seam line by applying an as low as possible reproducible defined bursting force. At the same time, the airbag cover should be able to offer the airbag module mechanical protection against any spontaneous shock that is not related to an accident, which is the reason why the necessary bursting force should not fall below a certain minimum.
In addition to these technical requirements, the predefined tear seam line should also meet certain esthetic requirements in that it remains invisible to the passengers even in the long term.
Predefined seam tear lines known in the prior art are formed by a continuous groove or a sequence of grooves or holes (blind holes or microperforation holes) or combinations of these. The depth to which material is removed may be uniformly constant or may vary along the length of the predefined tear seam line following a certain pattern.
In the explanations below, the term “pre-weakened line” will be used to describe linear material removal in only one layer of the material. A predefined tear seam line is formed by superimposing the pre-weakened lines in separate layers in the final layered structure of the interior lining component of the automotive vehicle, one on top of the other.
To create a pre-weakened line in the decorative layer, the above-mentioned types of predefined tear seam lines are used while the pre-weakened line in the support layer can also be formed by slits or through bores.
The fact that there are such a large number of patents that attempt to solve the problem of producing such a predefined tear seam line is evidence of the technical difficulties developers are facing when tackling this task. These technical difficulties are attributable especially to the different physical properties of the layered structure, which properties make it especially difficult to machine all of the layers in the same manner.
Based on the sequence of the process steps, manufacture of the layered structure and production of the predefined tear seam line, it is possible to divide the prior art methods into three groups:                Separate production of a pre-weakened line in the support layer and in the decorative layer and subsequent joining of the layers to create the layered structure of the composite component        Production of a pre-weakened line in the form of a gap or slit in the support layer, joining of the layers to create the layered structure and production of a pre-weakened line in the decorative layer through the gap or slit        Joining of the separate layers to create the layered structure and production of a predefined tear seam line        
In the method of the first mentioned group, pre-weakened lines are produced separately in the support material and in the decorative material. In this case, it is possible to choose different technological methods or different process parameters specifically suitable for each material. The problem encountered in this type of method is that the two pre-weakened lines must be produced so as to be congruent and must be joined so as to be exactly on top of each other in order to jointly form a predefined tear seam line of defined bursting force.
European Patent 0 711 627 A1 describes this type of method in which the pre-weakened line in the decorative layer is produced by means of a laser. It mentions that the depth of the groove must be carefully controlled in order to cause the decorative layer to rupture at precisely the right moment.
A method of the second above mentioned group is known from European Patent EP 0 967 066 B1. In this case, the support layer, is produced with a gap being integrated along the predefined tear seam line desired, with said gap possibly being interrupted by bridging elements. Subsequently, the support layer is joined indirectly or directly to the decorative layer, and the decorative layer is weakened through the gap. Tools mentioned as being suitable for this purpose are a laser, a mechanical knife, ultrasound, a hot knife or a water jet.
It is claimed that the special advantage of such a solution as compared to the third mentioned group of methods is the simultaneous formation of the gap during the manufacture of the support layer, which is said to make the subsequent step of removing material from the support layer superfluous. The advantages claimed are that the cycle time can be reduced, that a low-energy laser can be used, and that it is not necessary to take measures to handle the by-products that form during the evaporation of the support material.
Compared to the first group of methods, the advantage is that the decorative layer can be joined to the support layer, without having to align previously separately produced pre-weakened lines with respect to each other. Weakening the decorative layer through a gap existing in the support layer ensures that the two pre-weakened lines, which form the predefined tear seam line, in the support layer (produced by the gap) and in the decorative layer (produced by a sequence of holes) are congruent with respect to each other.
As in the first group of methods mentioned above, the laser parameters can be selected so as to be optimally suitable for the material of the decorative layer.
Bridging elements that bridge the gap are mentioned in European Patent EP 0 967 066 B1 as not being absolutely necessary but only as being useful. Such bridging elements can be incorporated while profiling the support layer comprising the gap or they can be attached to the support layer later so that they bridge the gap. The bridging elements are intended to stabilize the opening flap outlined by the gap against the surrounding support layer.
In the absence of bridging elements that bridge the gap, additional measures must be taken to protect the predefined tear seam line against accidental destruction due to spontaneous shock that is not related to an accident but instead is generated inside the passenger cabin. No information about such measures is provided.
When the slit is formed with bridging elements that bridge the gap, the decorative layer underneath the bridging element is by necessity not weakened if, as is the case here, a pre-weakened line is produced only in the slit regions. This can lead to uncontrolled tearing in the decorative layer if the predefined tear seam line in the unweakened regions below the bridging elements in the support layer is ruptured.
In the third group of methods mentioned, a predefined tear seam line is formed in the already prefabricated layered structure of an interior lining component from the side of the support layer. A tool especially suitable for this purpose was found to be a pulsed laser with its controllable laser parameters, pulse amplitude, pulse width and pulse frequency since it is possible, by way of changing many of the process parameters, to influence the effect of such a laser in a manner specifically suitable to the material used, without having to change tools. It is, however, also possible to use a continuous laser if the energy of the laser is appropriately modulated.
Only in exceptional cases is it possible to use constant laser parameters for all layers when machining a layered structure, such as it is commonly used for interior lining components.
In most cases, the laser energy required to penetrate through the support layer within a reasonable time would be far too high to maintain a possibly existing intermediate layer as intact as possible and to weaken the decorative layer so that only a minimum residual wall thickness remains. To ensure that the quality of the predefined tear seam line meets all demands, the laser parameters must be adjusted, via the depth to which material is removed, to the properties of the individual successive layers of the layered structure.
The depth to which material is removed and/or the residual wall thickness, at least in cases in which the depth to which material is removed extends into the decorative layer, is/are preferably monitored to ensure that a complete through penetration of the decorative layer is reliably avoided (except for visually imperceptible microperforation holes), thus ensuring that invisibility is maintained.
This type of method is described in European Patent EP 0 827 802 B1.
To carry out the method according to European Patent EP 0 827 802 B1, a laser beam—the pulse amplitude, pulse width and pulse frequency of which can be controlled—is directed at the layered structure from the side of the support layer. Underneath the layered structure, a sensor, which is oriented in the direction of the laser beam, is disposed on the side of the decorative layer, which sensor detects the portion of the radiation energy of the laser beam that is transmitted through the layered structure. The parameters of the detector and of the laser radiation source are coordinated with each other so that a first signal is detected when the support layer in the area exposed to the laser beam is completely removed. This assumes that the layers following the support layer are partly transparent for the wavelength of the machining laser beam.
Machining the layered structure begins with a pulse type with high pulse energies, which makes it possible to rapidly penetrate through the support layer. As the first laser beam is detected, which is possible after the complete through penetration of the support layer, the pulse type is changed in order to adjust it to the properties of the layer of the expanded plastic material and the decorative layer that determine the removal of material by the laser. Specifically, the pulse duration is shortened, which ensures lower thermal stress and gentle material removal.
European Patent EP 0 827 802 B1 shows that it was recognized as early as 1998, i.e., at the time of the present patent application, that it is possible, using a suitable laser regimen, to weaken a layered structure comprising materials with different physical properties and different layer thicknesses along a predefined tear seam line in a reproducible and predefined manner so that said predefined tear seam line can be ruptured by means of a predefined bursting force.
As practical experience has demonstrated, however, especially intermediate layers, which can be exposed to low thermal stress only, can be removed over large surface areas to the point that they disappear completely along the predefined tear seam line once the support layer has been pierced, even when the exposure to laser energy high enough to machine the support layer is of short duration only. The decorative layer, which is subsequently no longer supported along the predefined tear seam line, collapses—regardless of whether the pre-weakened line in the decorative layer is formed by blind holes or microperforation holes jointly referred to as holes)—and becomes visible.
An increase in the spacing between the holes along the entire length of the predefined tear seam line to ensure that a supporting bridging element is securely maintained between the holes in the intermediate layer would excessively increase the required bursting force.
It will be obvious to the person skilled in the art that the effect will be the same if, instead of a soft intermediate layer and a thin compact decorative layer, a decorative layer is used, which combines the function of the conventional intermediate and decorative layers mentioned.
German Patent DE 198 19 573 A1 discloses an instrument panel on which, along the length of a predefined tear seam line, blind holes are formed by means of a laser, which blind holes completely penetrate through a support layer and a superimposed intermediate layer and end within the outer decorative layer. A preformed groove can be present along the predefined tear seam line in the support layer.
In contrast to the patent just mentioned above, in the method disclosed in German Patent DE 102 27 118 A1, the energy input for the individual blind holes is periodically varied so as to obtain, on the one hand, blind holes which penetrate only into or through the support layer and, on the other hand, extend into the decorative layer. As a result, bridging elements that serve to support the superimposed decorative layer are retained in the intermediate layer.
German Patent DE 10 2004 047 634 A1 discloses an instrument panel having the same layered structure as well as a method of producing said instrument panel, with a predefined tear seam line made of blind holes of different residual wall thickness.
International Publication WO 99/01317 A1 also describes an airbag cover and a method of manufacturing it, which method, like the ones disclosed in the documents mentioned above, uses a laser to machine holes into a layered structure comprising a support layer, an intermediate layer and a decorative layer. The holes can extend to varying depths and, in this case, sometimes even completely penetrate through the decorative layer.
In all of the documents mentioned, a support layer is not only present [sic] along the desired predefined tear seam line but, in its thickness, is also constant along its length, or at least along the length of the sections that are to be weakened.